Recently, mud and snow tires to be used in, for example, 4 WD (four-wheel-drive) vehicles are required to have an improved high speed durability because such vehicles are greatly improved in their performance and the maximum running speed is increased. For example, there is a great demand for a mud and snow tire being capable of satisfying S-range (ECE30, 180 km/h-20 minutes) or H-range (ECE30, 200 km/h-20 minutes).
It is preferable to decrease the rubber thickness in the shoulder portion of a tire from the point of view of high speed durability. Indeed, this has been originally employed in passenger tires to improve high speed durability.
In the mud and snow tires, however, a relatively deeper groove depth is required in contrast to the passenger tires. In other words, relatively higher blocks are required. And in order to support such blocks, a sufficient thickness is required for the under tread rubber. Thus, it is very difficult to decrease the rubber thickness to improve the high speed durability.